Not too easy to see, but I suppose you want to spread your fingers more than you can do right now.
Well, spread them when you have the time, no aids, just spread. You can, *very* carefully and not too often, use a table to hook up a finger on and *gently* push downwards.
And of course, play an awful lot, 'cuz practice does help this.
Practice also will give you the ability to pivot on your thumb. This will probably add around 3 or 4 frets of reach, within reasonable time.

play semitones on every string from the first to fourth fret, but don't lift your fingers off the string when skipping to the next one. G string: G#, A, A#, B. D string: D# (keep your other fingers on the G string), E, F, F#. A string: A# (keep your fingers on the D string), B, C, C# and so on. Play quarter notes with a metronome at 60 beats per minute. Take it easy and don't hurt yourself. With time and practice, you'll gain endurance and finger stretching.

P.S.: "Another one bites the dust" is originally in open E. The false D# on "Portrait of Tracy" is a really big hand stretching!

This isn't really a bassline, but try playing the guitar part to the Police's "Message in a Bottle". That should stretch your fingers pretty good. And even though this song consists of only a few notes, "Green Onions" by Booker T. and the MGs has an F, G#, Bb reach as well.